Fully integrated tuneable spin torque device for generating an oscillating signal and method for tuning such apparatus

ABSTRACT

The present invention is related to a device and corresponding methods for generating an oscillating signal. The device comprises a means for providing a current of spin polarised charge carriers, a magnetic, e.g. ferromagnetic, excitable layer adapted for receiving the generated current of spin polarised charge carriers thus generating an oscillating signal with a frequency V osc  and an integrated means for interacting with said magnetic, e.g. ferromagnetic, excitable layer such that a selection of said oscillation frequency is achieved. No external field needs to be applied to select or tune the frequency. Different types of integrated means can be used, such as e.g. means inducing mechanical stress in the magnetic, e.g. ferromagnetic, excitable layer, means inducing exchange bias interactions and means inducing magnetostatic interactions.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to devices and corresponding methods forgenerating oscillating signals such as e.g., but not limited to, currentinduced oscillators, modulators and filters, which are based on the‘spin torque’ effect.

BACKGROUND OF THE INVENTION

Modern RF front ends in portable wireless devices, such as portablewireless communication devices, require minimal size, stable, widebandtuneable oscillators with low operating power and high output power. Thefrequencies used in commercial systems today are synthesized fromoff-chip passive resonators, e.g. quartz crystals, with a veryhigh-quality resonance at low frequencies (10-30 MHz). The quality ofthe oscillation is typically represented by its quality factor Q definedby the ratio of the peak frequency of the oscillation peak to the widthof the oscillation peak at half of the maximum amplitude. In FIG. 1 therelation between the oscillation peak and the quality factor Q is shown.This typically is about 10000 for quartz oscillators. Such oscillatorshave lateral dimensions on the order of 1 or 2 mm. Integratedoscillators on the other hand, which may e.g. consist of an LC-tankcircuit or a miniaturized integrated resonators (RF-MEMS) typically havea much lower quality factor. For integrated oscillators consisting of anLC-tank circuit the quality factor Q typically is smaller than 100,whereas for miniaturized integrated mechanical resonators a qualityfactor larger than 1000 can be obtained at high frequencies (500 MHz-6GHz). Nevertheless, the latter require complicated fabrication steps andit is difficult to tune their frequency.

Some applications of the spin torque effect are known. Some aspects ofthe spin torque effect have been predicted and described in 1996 in forinstance patent U.S. Pat. No. 5,659,864. Demonstrations of spin torquehave so far been focused on ‘current-induced switching’ for applicationin Magnetic Random Access Memories. Some examples of the use of spintorque effects are listed below. In S. I. Kiselev et al., Nature 425,380 (2003), the spin torque effect is studied in a basic structure beinga multilayer of 80 nm Cu/40 nm Co/10 nm Cu/3 nm Co/2 nm Cu/30 nm Ptpatterned into a nanopillar (elliptical cross section of 130 nm×70 nm)and contacted with a Cu top contact. The 40 nm Co layer acts as thefixed layer, the 10 nm Cu layer is the interlayer and 3 nm Co layer isthe excitable layer. The nanopillar embodiment allows to obtain aconfined spin polarised charge carrier current.

In W. H. Rippard et al. PRL 92, 027201 (2004), the spin torque effect isstudied in a basic structure being a CoFe/Cu/NiFe trilayer that wascontacted by patterning a nanohole (diameter 40-100 nm) in the isolatoron top. Oscillations were obtained with a frequency that was tuneablebetween 5 and 35 GHz, depending on the value of the external magneticfield. The modes of most interest are those with very high Q-factoroscillations. The largest observed Q of 18000 was obtained in a nanoholefor a magnetic field of 1 T applied at an angle of 30° from the surfaceand a current of 6 mA, resulting in a frequency of 35.4 GHz. Reasonablyhigh quality factors, but with higher output powers, could be reached atlow magnetic fields, e.g. Q=2705 at 0.15 T applied at 85° to thesurface, corresponding to a frequency of 9.69 GHz. FIG. 2 a illustratesthe frequency spectrum of the AC voltage measured over a trilayercontacted through a 40 nm nanohole when a current of 6 mA is sentthrough and an external field H of 0.15 T is applied at an angle of 85degrees from the surface, as indicated in the inset. The peak frequencycan be set by the external magnetic field in a broad range of 5-35 GHzand with a slope of 26.2 GHz per Tesla as seen in FIG. 2 b. The currentcan also be used to set and control the frequency with a slope of −0.23GHz per mA, as shown in FIG. 2 c.

Some basic geometries that are used in the state-of-the-art geometriesof the current induced oscillator tunable by an external magnetic biasfield H, are shown by the devices 100 in FIG. 3. The external magneticbias field H might be oriented along any direction. Two typicalexamples, i.e. the nanopillar embodiment and the nanohole embodiment,both allowing confinement of the current, are illustrated in FIG. 3. Thediameter of the nanopillar or nanohole is typically small to guarantee ahigh degree of confinement of the DC current. The systems comprise aso-called trilayer, which exists of a ferromagnetic exitable layer 102,an interlayer 110 and a fixed layer 112. This trilayer might bepatterned into a nanopillar or contacted through a nanohole to confinethe current and increase the spin torque exerted on the excitable layer.Current is applied through electrodes 116.

The external, i.e. not integrated or united, electromagnet that iscurrently used requires external not-integrated components. This makesthe device large in size, heavy and unpractical.

There is a need to provide fully integrated RF circuits, in which allessential elements for selecting a frequency or tuning a frequency ofthe oscillations are present in a united way, i.e. structurally orfunctionally, and which furthermore are able to set or tune the peakfrequency of the oscillation while keeping the quality factor and thestability of the oscillation maximal, i.e. as high as possible, and withminimal additional power consumption.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide devices and methodswhich solve at least some of the problems of the prior art. At least oneof these problems is:

integration of the components to obtain compact devices,

obtaining tunability combined with keeping the quality factor and thestability of the oscillation maximal and with keeping minimal additionalpower consumption.

It is also an object of the present invention to provide novel methodsand corresponding devices for setting the frequency of a current inducedoscillator with high controllability.

It is furthermore an object of the present invention to providealternative read out schemes for an oscillator.

The above objectives are accomplished by a method and device accordingto the present invention.

The invention relates to a device for generating an oscillating signal,the device comprising a means for providing a current of spin polarisedcharge carriers, a magnetic excitable layer adapted for receiving saidcurrent of spin polarised charge carriers thus generating an oscillatingsignal with a frequency ν_(osc) and an integrated means for interactingwith said magnetic excitable layer such that a selection of saidoscillation frequency is achieved. The magnetic excitable layer may be aferromagnetic excitable layer. Said integrated means for interactingwith said magnetic excitable layer such that a selection of saidoscillation frequency is achieved may be a means for controllablytunable interacting with said magnetic excitable layer such that acontrollable tuning of said oscillation frequency is achieved. Saidinteracting may comprise performing magnetic interactions comprisinginducing mechanical stress in said magnetic excitable layer. Saidmagnetic interactions may be interface interactions. Said surfaceinteractions may comprise exchange bias interactions. Said interactingmay comprise performing magnetostatic interactions. Said magneticexcitable layer may be a ferromagnetic semiconductor layer and saidinteracting may comprise applying an electric field over saidferromagnetic semiconductor layer. Said device may comprising a meansfor generating a magnetic bias field to bias the magnetic excitablelayer. Said means for generating a magnetic bias field may be an antiferromagnetic layer which is in at least partial magnetic contact withsaid magnetic excitable layer. The device may comprise a means forgenerating stress upon said anti ferromagnetic layer. The means forgenerating said magnetic bias field may comprise an element offerromagnetic material which is magnetostatically coupled to saidmagnetic excitable layer. The device may comprise a means for changingthe geometric distances between said magnetic excitable layer and saidferromagnetic element. The means for changing the geometric distancesmay consist of a piezoelectric layer or suspended structure. The meansfor changing the geometric distances may e.g. comprise a cantileverstructure. Said integrated means for interacting with said magneticexcitable layer may comprise an interacting layer, which is coupledmagneto-elastically and/or magneto-statically and/or via the exchangebias effect to said magnetic excitable layer. Said interacting layer maybe a piezoelectric layer and/or an antiferromagnetic layer. The devicefurthermore may comprise a surface acoustic wave generating means whichcan generate a Surface Acoustic Wave in said interacting layer. Saidinteracting layer may be a structural part of the Surface Acoustic Wavegenerating means. Said Surface Acoustic Wave generating means maygenerate a Surface Acoustic Wave in said interacting layer, which has afrequency essentially equal to the magnetic resonance frequency of saidexcitable layer, or an integer multiple thereof. At least 2 electrodesmay be provided on a surface of or inside said interaction layer, whichallow to induce stress in said interaction layer by putting anelectrical potential difference over them. The device may comprise ameans for generating stress in said interaction layer by physical forceor pressure build up. The means for providing a current of spinpolarised charge carriers may be abutting on said magnetic excitablelayer and may comprise an electrode, a spin polarisation means and acurrent confinement structure. The means for providing a current of spinpolarised charge carriers may comprise a fixed layer with a constantmagnetic polarisation through which the current is passing, beforeentering into the excitable layer. The fixed layer and excitable layermay be separated by an interlayer to magnetically separate both layers.The device may comprise a readout structure, which measures theexcitation caused by the spin polarised current passing through saidmagnetically excitable layer or a related or equivalent parameter. Thedevice may comprise a readout structure, which measures themagneto-resistance or a related effect, generated by combination of thefixed layer and the magnetic excitable layer. The device may comprise areadout structure, which comprises a piezoelectric measurement layer,which converts the precessional movement of the excitable layer into anelectrical signal. The device also may comprise a readout structure,which measures the resistance change by measuring the AC signal betweenat least 2 electrodes in electrical contact with said excitable layer.The device may comprise a readout structure, which measures the changeof resistance or voltage in a lateral geometry.

The invention also relates to a method for generating oscillations, themethod comprising providing a current of spin polarised charge carriers,thus generating an oscillating signal with an oscillation frequencyν_(osc) by interaction between said current of spin polarised chargecarriers and a magnetic excitable layer and controllably tuning saidoscillation frequency ν_(osc) by inducing an interaction between anintegrated means and said magnetic excitable layer. The magneticexcitable layer may be a ferromagnetic excitable layer. Inducing aninteraction between an integrated means and said magnetic excitablelayer may comprise any of inducing mechanical stress in said magneticexcitable layer, inducing exchange bias interactions and inducingmagnetostatic interactions. Said magnetic excitable layer may be aferromagnetic semiconductor layer, and inducing an interaction may beperformed by applying an electric field over said ferromagneticsemiconductor layer. Said inducing exchange bias interactions maycomprise generating a magnetic bias field to bias the excitable layer.Generating a magnetic bias field may comprise generating stress upon ananti ferromagnetic layer in magnetic contact with said magneticexcitable layer. Inducing an interaction may comprise bringing aferromagnetic element in contact with the magnetic excitable layer andchanging the geometric distance between said element and said layer.Inducing an interaction may comprise generating a surface acoustic wavein an interacting layer provided in the neighbourhood of the magneticexcitable layer, wherein the surface acoustic wave has a frequency thatis substantially equal to the magnetic resonance frequency of saidexcitable layer, or an integer multiple thereof. Inducing an interactionmay comprise generating stress in an interacting layer in direct orindirect contact with the magnetic excitable layer. Said generatingstress may be performed by applying an electric field to saidinteracting layer, said interacting layer e.g. being a piezoelectriclayer. Said generating stress may be performed by exerting physicalforce or pressure on said interaction layer. Said providing a current ofspin polarised charge carriers may comprise providing a confined currentof spin polarised charge carriers.

The invention also relates to a method for reading out a magneticelement, the method comprising providing a current of spin polarisedcharge carriers, thus generating an oscillating signal with anoscillation frequency ν_(osc) by interaction between said current ofspin polarised charge carriers and a magnetic excitable layer,controllably tuning said oscillation frequency ν_(osc) by inducing aninteraction between an integrated means and said magnetic excitablelayer and measuring an excitation, or a related or equivalent parameter,said excitation being caused by said spin polarised charge carriers. Themagnetic excitable layer may be a ferromagnetic excitable layer.Measuring an excitation or a related or equivalent parameter caused bysaid spin polarised charge carriers may comprise converting theprecessional movement of the excitable layer into an electrical signal.Measuring an excitation or a related or equivalent parameter caused bysaid spin polarised charge carriers may comprise measuring a resistancechange by measuring an AC signal between at least 2 electrodes inelectrical contact with said magnetic excitable layer. Measuring anexcitation or a related or equivalent parameter caused by said spinpolarised charge carriers may comprise measuring the change ofresistance or voltage in a lateral geometry.

It is advantageous that the present invention is based on the spintorque effect. This effect refers to the effect whereby a torque isexerted by a spin-polarized current of charge carriers, e.g. an electronflow, on a thin magnetic excitable layer. The spin-polarized currentcreates a stable magnetic precession by a torque that counteracts theintrinsic damping of magnetic motion of the excitable layer. The currentcan thus generate high-quality microwave oscillations. Such devices canfor instance, but not only, be used as an RF oscillator, clock,modulator in e.g. microwave transceivers and integrated RF circuits.

It is also an advantage of the present invention to provide a means thatcan controllably alter the frequency by indirect interaction with theexcitable layer through a means for generating the magnetic bias field.

The present invention provides methods of biasing and tuning a spintorque oscillator by providing an integrated means for tunableinteracting directly or indirectly with the excitable layer throughmagnetic interactions such as magneto-elastic and/or magneto-staticcouplings and/or exchange bias effects.

For the purpose of the present invention, the term “magnetostaticcoupling” comprises the mutual influence of magnetic materials whenplaced close to each other and is further defined by the state of theart. The energy associated with this coupling is called “magnetostaticenergy”.

For the purpose of the present invention, the term “magnetoelasticcoupling” relates to the effect whereby strain and stress in aferromagnetic material changes the magnetocrystalline anisotropy and maythereby influence the magnetization state. The coupling effects ofstress and strain (coupled by Young's modulus) in a material and itsmagnetic properties are often called and comprised in the term“magneto-elastic coupling” and the energy associated with this effect iscalled the “magnetoelastic energy”.

Particular and preferred aspects of the invention are set out in theaccompanying independent and dependent claims. Features from thedependent claims may be combined with features of the independent claimsand with features of other dependent claims as appropriate and notmerely as explicitly set out in the claims.

Although there has been constant improvement, change and evolution ofdevices in this field, the present concepts are believed to representsubstantial new and novel improvements, including departures from priorpractices, resulting in the provision of more efficient, stable andreliable devices of this nature.

The teachings of the present invention permit the design of improvedmethods and apparatus for generating oscillating signals in compactdevices.

These and other characteristics, features and advantages of the presentinvention will become apparent from the following detailed description,taken in conjunction with the accompanying drawings, which illustrate,by way of example, the principles of the invention. This description isgiven for the sake of example only, without limiting the scope of theinvention. The reference figures quoted below refer to the attacheddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the relation between the quality factor Q and theoscillation peak obtained in a device for generating oscillations, asknown.

FIG. 2 a —prior art to FIG. 2 c prior art shows microwave oscillationwith a high quality factor Q in a system known from the prior art. FIG.2 a is a graph of the power as a function of the frequency, FIG. 2 bindicates the tunability of the peak frequency as a function of theexternal magnetic field and FIG. 2 c indicates the peak frequency as afunction of the DC current used in the system.

FIG. 3—prior art shows basic geometries as currently used in currentinduced oscillators known from the prior art. Both a nanopillarpatterned trilayer and a system for contacting through a nanohole areillustrated.

FIG. 4 shows a schematic diagram of the different components of anelectronic device for generating oscillations with a selectablefrequency according to the first embodiment of the present invention.

FIG. 5 shows a schematic representation of an integrated structure foran electronic device for generating oscillation with a selectablefrequency according to the first embodiment of the present invention.

FIG. 6 a to FIG. 6 c shows a schematic representation of an integratedstructure for an electronic device for generating oscillation with aselectable frequency based on biasing of the magnetic excitable layeraccording to the second embodiment of the present invention. FIG. 6 aillustrates alternative configurations of an embodiment with aninteracting layer being an anti-ferromagnetic material in direct contactwith the magnetic excitable layer, FIG. 6 b illustrates an embodimentwith an interacting layer being a ferromagnetic material in magneticcontact with the magnetic excitable layer.

FIG. 7 to FIG. 10 b are schematic representation of differentalternative configurations for an integrated structure for an electronicdevice for generating oscillation with a tunable frequency, said tuningbeing based on generating stress in piezoelectric material, according toa third embodiment of the present invention. FIG. 8 c therebyillustrates the stress dependence of an antiferromagnetic material e.g.in the embodiments shown in FIG. 8 a and FIG. 8 b.

FIG. 11 a and FIG. 11 b are schematic represenations of an integratedstructure for an electronic device for generating oscillation with atunable frequency, said tuning being based on electrically influencing aferromagnetic semiconductor layer

FIG. 12 is a schematic representation of an integrated structure for anelectronic device for generating oscillation with a tunable frequency,said tuning being based on generation of stress by introducing a surfaceacoustic wave, according to the sixth embodiment of the presentinvention.

FIG. 13 a and FIG. 13 b are schematic representations of an integratedstructure for an electronic device for generating oscillation with atunable frequency, said tuning being based on changing the magnetostaticcoupling, according to the seventh embodiment of the present invention.

FIG. 14 a to FIG. 14 b is a schematic representation of a deviceallowing a first alternative read out based on measuring a lateralresistance according to a further embodiment of the present invention.FIG. 14 a illustrates a side view, FIG. 14 b and FIG. 14 c illustrates atop view for two different alternatives

FIG. 15 is a schematic representation of a device allowing anotheralternative read out based on measuring an electrical signal in apiezoelectric layer created by the precessional motion in the magneticexcitable layer, according to a further embodiment of the presentinvention.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The present invention will be described with respect to particularembodiments and with reference to certain drawings but the invention isnot limited thereto but only by the claims. The drawings described areonly schematic and are non-limiting. In the drawings, the size of someof the elements may be exaggerated and not drawn on scale forillustrative purposes. The dimensions and the relative dimensions do notcorrespond to actual reductions to practice of the invention.

Furthermore, the terms first, second, third and the like in thedescription and in the claims, are used for distinguishing betweensimilar elements and not necessarily for describing a sequential orchronological order. It is to be understood that the terms so used areinterchangeable under appropriate circumstances and that the embodimentsof the invention described herein are capable of operation in othersequences than described or illustrated herein.

Moreover, the terms top, bottom, over, under and the like in thedescription and the claims are used for descriptive purposes and notnecessarily for describing relative positions. It is to be understoodthat the terms so used are interchangeable under appropriatecircumstances and that the embodiments of the invention described hereinare capable of operation in other orientations than described orillustrated herein.

It is to be noticed that the term “comprising”, used in the claims,should not be interpreted as being restricted to the means listedthereafter; it does not exclude other elements or steps. It is thus tobe interpreted as specifying the presence of the stated features,integers, steps or components as referred to, but does not preclude thepresence or addition of one or more other features, integers, steps orcomponents, or groups thereof. Thus, the scope of the expression “adevice comprising means A and B” should not be limited to devicesconsisting only of components A and B. It means that with respect to thepresent invention, the only relevant components of the device are A andB.

The invention will now be described by a detailed description of severalembodiments of the invention. It is clear that other embodiments of theinvention can be configured according to the knowledge of personsskilled in the art without departing from the true spirit or technicalteaching of the invention, the invention being limited only by the termsof the appended claims.

In a first embodiment, the present invention relates to an electronicdevice 200, also referred to as a tuneable current-induced oscillator,comprising a magnetic excitable layer 202, e.g. ferromagnetic excitablelayer 202, a means for providing a current of spin polarised chargecarriers 204 into the excitable layer 202 such that oscillations aregenerated in the ferromagnetic excitable layer 202 with a frequencyv_(osc) and a means for interaction 206 with the ferromagnetic excitablelayer as to select a frequency of the generated oscillations. See FIG.4. For clarity reasons in the following a ferromagnetic excitable layer202 will be discussed further, although the invention is not limited toferromagnetic excitable layers but refers to all magnetic excitablelayers. In a preferred embodiment the means for interaction 206 with theferromagnetic excitable layer as to select a frequency of the generatedoscillations may be a means for interaction 206 with the ferromagneticexcitable layer as to tune a frequency of the generated oscillations.The latter implies that the frequency can not only be selected duringdesigning or production of the device but that the frequency also can beselected during use, i.e. that the frequency can be tuned during use. Aschematic representation of the different components of the device 200and the interaction between the different components is illustrated inFIG. 4. It is an advantage of the embodiments of the present inventionthat novel methods and corresponding devices are provided for settingthe frequency of a current induced oscillator with high controllability.

The excitable ferromagnetic layer 202 may be a thin ferromagnetic filmof any alloy of Co, Fe, Ni, or another ferromagnetic metal or asemiconductor with ferromagnetic properties such as Mn-doped GaAs or Crdoped GaN, AlN, etc . . . The means for providing a current of spinpolarised charge carriers 204 may be any means for providing such acurrent which allows to obtain a high spin polarised charge carriersdensity. An example of such a means may be a trilayer, existing of afixed ferromagnetic layer, also called a spin polarising layer, aninterlayer and a ferromagnetic excitable layer, which in the presentinvention is the excitable ferromagnetic layer 202. As the ferromagneticexcitable layer 202 has a specific separate function, in the presentapplication it is considered not to be a part of the means for providinga current of spin polarised charge carriers 204, for clarity reasons.Trilayers are well known by the person skilled in the art. In anembodiment wherein the means for providing a current of spin polarizedcharge carriers 204 as e.g. shown in FIG. 5, the interlayer 210 may be athin non-ferromagnetic metal such as Cu, a thin isolating tunnel barriersuch as Al₂O₃ or MgO or a semiconducting non-ferromagnetic layer andshould efficiently transfer the spin from the fixed layer 212 to theferromagnetic excitable layer 202, also called the free layer. The fixedferromagnetic layer 212 may be a thin ferromagnetic film of any alloy ofCo, Fe, Ni, or another ferromagnetic metal or a semiconductor withferromagnetic properties. A tuneable oscillator based on such a trilayercan be made e.g. by patterning a nanopillar in a multilayer stack ore.g. by electrodepositing a nanopillar in a template, or can be madee.g. by providing a nanocontact through a hole in an insulator 214, asshown in FIG. 5. The nanopillar or nanohole cross section can becircular, elliptical, rectangular or any other cross section. Thediameter of the nanopillar or nanohole is typically small, e.g. between5 and 500 nm, as to confine the current to a small volume to get a highspin torque efficiency with little power dissipation, which is relatedto the DC current amplitude. The trilayer may be fabricated with fixedlayer closest to the substrate or with excitable layer closest to thesubstrate. The current typically is applied using a first and a secondelectrode 216. The means for interaction 206 with the ferromagneticexcitable layer as to select a frequency of the generated oscillations,or preferably as to tune a frequency of the generated oscillations maybe applied in a large number of ways. Use may be made of a means forproviding mechanical stress in the ferromagnetic excitable layer 202,exchange bias interactions with the ferromagnetic excitable 202 layer,magnetostatic interactions with the ferromagnetic excitable layer 202.Several specific examples will be described in further embodiments,although the invention is not limited thereto. By way of example, inFIG. 5 an interacting layer 218 e.g. generating stress or inducingmagnetostatic or exchange bias interactions is provided in FIG. 5.Possible additional means for differing the amount of generated stress,the amount of induced magnetostatic interactions or the amount ofexchange bias interactions are not illustrated. The selecting or tuningof the frequency of the generated oscillations may be performed withrespect to a maximum oscillation frequency, although the invention isnot limited thereto.

Additional layers may be added to the device in the ferromagneticexcitable layer 202 or the ferromagnetic fixed layer 212 or at theinterface with the interlayer 218 to provide a higher magnetoresistiveeffect and thus a high output power for the oscillator for lowercritical currents. The addition of layers to increase themagnetoresistance follows the ideas and concepts that have often beenused in the magnetic recording industry to increase the magnetoresistivesignal of a recording head. Such a recording head is typically a spinvalve or a magnetic tunnel junction. They consist basically of atrilayer, ferromagnetic free layer, interlayer, ferromagnetic fixedlayer, where the magnetoresistive signal originates from thespin-dependent scattering or spin-dependent tunnelling of the readcurrent, and is strongly dependent on the angle between the fixed andthe free layer. Examples of additional layers are materials with highpolarisation of charge carriers added at the interfaces of theferromagnetic layers and the interlayer to increase magnetoresistance,adding very thin layers, such as nano-oxyde layers to reflect spincurrents thereby increasing the degree of spin polarisation.

Additional layers may also be added to pin the fixed layer while notcreating unwanted magnetostatic couplings to the free or excitablelayer. A typical method used in the recording industry to provide thepinning is to bring the fixed layer into contact with anti-ferromagneticlayer such as FeMn, and IrMn, and due to a magnetic interfaceinteraction called exchange bias the hysteresis loop of the fixed layeris shifted towards higher fields. Exchange bias is a phenomenon thatresults because the spins of the ferromagnetic and anti-ferromagneticmaterial at the interface have to aligned in a compatible way. The fixedlayer may also consist of a synthetic ferromagnetically coupledtrilayer, consisting of two ferromagnetic layer that are separated by asecond interlayer, being a non magnetic layer. The interlayer separatesthe fixed layer into two fixed layers of opposite magnetisation thatform an almost closed magnetic path and therefore result in suppressionof the unwanted magneto-static stray-fields. The stack of layers thene.g. comprises a free ferromagnetic layer/a non magnetic layer/a fixedferromagnetic layer/a non magnetic layer/a fixed ferromagnetic layer/andan anti-ferromagnetic layer.

Furthermore, other layers could be added to further increase the amountof spin filtering, e.g. mirroring the structure to create a so calleddouble spin filter with a excitable layer sandwiched between both. Suchlayers are known from e.g. US 2004/0208053. Any extra layers can beadded known to a person of ordinary skill in the art.

The generation of a current of spin polarised charge carriers will nowbe discussed in more detail, by way of example for an embodiment whereinthe means for providing a current of spin polarised charge carriers 204into the ferromagnetic excitable layer 202 comprises a trilayer. It isto be noted that the invention is not limited thereto. In general ameans for providing a current of spin polarised charge carriers 204 maybe a means wherein a current is sent through a ferromagnetic material.The trilayer, consists of an excitable ferromagnetic layer 202, aninterlayer 210 and a fixed ferromagnetic layer 212 with layer thicknessof about 0.5-500 nm for each layer, where the free or excitableferromagnetic layer 202 is typically thinner or has a lowermagnetisation or a lower damping parameter of magnetic motion than thefixed ferromagnetic layer 212, such that the precessional motionsinduced by the current is larger in the excitable ferromagnetic layer202 than in the fixed ferromagnetic layer 212. The fixed ferromagneticlayer 212 acts then as a spin filter of the current sent through it, andthe spin-polarized current can then create a stable precessionaloscillation in the excitable layer. The current in the oscillator istypically confined to a small region to have a high enough currentdensity and high enough spin torque to create a considerable oscillationamplitude.

The precessional motion of the ferromagnetic excitable layer 202 that isdriven by the spin-polarized DC current is typically translated into anAC voltage because of the magnetoresistance present in the trilayer. Theprecessional motion of the ferromagnetic excitable layer 202 results ina varying angle between the magnetisation orientation of the excitablelayer and the magnetisation orientation of the fixed layer andtherefore, due to spin-dependent scattering or spin-dependent tunnellingof the DC current, a magnetoresistive output signal at the frequency ofthe precessional motion of the ferromagnetic excitable layer 202results. The frequency is typically close to the ferromagnetic resonancefrequency of the ferromagnetic excitable layer 202 and can be between0.5 and 10 GHz depending on the materials used and the value of theexternal bias field.

In a second embodiment of the present invention, the present inventionrelates to a device as described in the first embodiment, wherein themeans for interaction 206 with the ferromagnetic excitable layer as toselect/tune a frequency of the generated oscillations is a means thatcan set the frequency by biasing the ferromagnetic excitable layer. SeeFIGS. 6 a, 6 b and 6 c. The biasing can be provided by bringing theferromagnetic excitable layer in physical contact or in partial physicalcontact with an interacting layer which is an anti-ferromagnetic layer222, as shown in the device 220 in FIG. 6 a. Through the exchange biaseffect, which is a phenomenon that results because the spins of theferromagnetic and anti-ferromagnetic material at the interface have toaligned in a compatible way, the hysteresis loop of the ferromagneticexcitable layer 202 will shift to higher external bias fields, such thata smaller or no external bias field is needed anymore to bias to set thefrequency and to reach the high quality factors obtained before by usingan external magnetic bias field. The anti-ferromagnetic material may bea metal e.g. IrMn, FeMn . . . or an isolator e.g. Cr02. If theanti-ferromagnetic material is metallic, it may be part of the stackthat is traversed by the current and the Joule heating by the currentmay reduce the exchange bias field, because the exchange bias field isstrongly temperature dependent. The anti-ferromagnetic material may alsobe in physical contact with the excitable layer without being a part ofthe stack traversed by the current. In that case the anti-ferromagneticmaterial may also be the isolator 214 that is used for the definition ofthe nanohole. See FIG. 6 b.

The biasing can also be provided by an additional ferromagnetic material224 that is part of the device and that is magnetostatically coupled (inmagnetic contact) to the ferromagnetic excitable layer 202. See FIG. 6c. The additional ferromagnetic material 224 does not have to be, butcan be, in physical contact with the ferromagnetic excitable layer buthas to be close enough to the ferromagnetic excitable layer 202 as toprovide magnetic flux generated by the additional ferromagnetic material224 to penetrate the ferromagnetic excitable layer 202, as shown inFIGS. 6 a and 6 b. The additional ferromagnetic material 224 may belying under or next to the current induced oscillator. The additionalferromagnetic material 224 may also be the isolator that is used for thedefinition of a nanohole.

In a third embodiment of the present invention, the invention relates toa device as described in any of the previous embodiments, wherein themeans for interaction 206 with the ferromagnetic excitable layer as toselect/tune a frequency of the generated oscillations is a means thatallows to controllably tune the frequency of the generated oscillations.This may happen by direct interaction with the excitable layer as tochange its magnetic properties. The means that interact with theexcitable layer may e.g. comprise a piezoelectric material, as indicatedin the device 230 of FIG. 7 and FIG. 8. The piezo-electric layer 232 canbe in contact or not in contact with the ferromagnetic excitable layer202, and when subject to an electrical field, the piezo-electricmaterial 232 will apply a stress on the piezo-electric layer 232 as wellas on the ferromagnetic excitable layer 202. The magnetic properties ofthe ferromagnetic excitable layer, i.e. the magnetic anisotropy and thegyromagnetic precession ratio or the Lande g-factor, and the magneticdamping parameter or Gilbert damping parameter, can be changed due tothe applied stress via magneto-elastic coupling. The change of magneticproperties results in a change of the oscillation frequency and thequality factor of the oscillation. The present invention can thereforebe used to tune the frequency of the oscillator in a controlled way bysetting the value of the stress by selecting the electrical field on thepiezoelectric material 232. One or more additional electrodes 234 may beprovided to apply the electrical field that generates the stress, on thepiezoelectric material 232. These additional electrodes 234 may belocated symmetrically around the oscillator to apply a potentialdifference between the electrodes, leaving the oscillator at a potentialin between the potential of the electrodes, or to apply a potentialdifference between the electrodes 234 and a contact to the oscillator.The piezoelectric material may also be located between the oscillatorand a bottom contact, such that the piezoelectric material is operatingin the so called d₃₁ operation regime, where the electric field isapplied on the material in its thickness direction and the stress/straincan be along the length and width directions. The latter is illustratedin FIG. 8 b. An example of the induced exchange bias field illustratingthe stress dependence of an anti ferromagnetic material is shown in FIG.8 c. The piezoelectric material 232 may be a piezoelectric substrate, apiezoelectric thin film layer or multilayer deposited on top of asubstrate, as shown in FIG. 9, a piezoelectric membrane or suspendedstructure with electrodes to stress the layer, as shown in FIG. 10 a,and may through magneto-elastic interaction change the properties of theferromagnetic excitable layer 202 thereby changing the oscillatorfrequency. The piezoelectric material 232 and the ferromagneticexcitable layer 202 may be in direct contact. The piezoelectric material232 may be the isolator in which the nanohole is patterned, as showne.g. in FIG. 10 b. The piezoelectric material 232 and the ferromagneticexcitable layer may also be one and the same material that arepiezoelectric and ferromagnetic at the same time, such as is the case ina class of materials named multiferroics.

In a fourth embodiment, the present invention relates to a device asdescribed in any of the previous embodiments, wherein the ferromagneticexcitable layer comprises a magnetic semiconductor. Two possibleconfiguration 250 are shown in FIG. 11 a and FIG. 11 b. In thisembodiment, the application of an electrical field to the semiconductorferromagnetic excitable layer 252 influences the magnetic propertiesusing the electrical properties of the semiconductor ferromagneticexcitable layer 252. In such a magnetic semiconductor such as Mn dopedGaAs, a large interplay exists between the electrical and magneticproperties. The interaction of the electrical field with the magneticsemiconductor 252 can create a reordering of spin carrying charges thatmay change the magnetic properties of the excitable layer locally orover the whole layer such that the frequency of the oscillation istuneable by the electrical field.

In a fifth embodiment, the present invention relates to a device asdescribed in any of the previous embodiments, wherein the means ofmagneto-elastic interaction with the ferromagnetic excitable layer 202consisting of a suspended structure that is actuated not by the electricfield and the piezoelectric effect but by another physical force, suchas electrostatic, i.e. capacitive, actuation by a potential differenceover a capacitor in which the suspended structure forms one of itselectrodes. The electric device then may have a similar structure as theone shown in FIG. 10. The present invention may also relate to anyinteraction layer that can interact magneto-elastically with theexcitable layer by generating a stress in said interaction layer. Theinteraction layer may consists of a suspended structure that can beactuated by a physical force such as temperature, laser light,mechanical force or by pressure build up.

In a sixth embodiment, the present invention relates to a devicecomprising similar features as described in the previous embodiments,but wherein the means for tuning comprises a piezoelectric layer and asurface acoustic wave generator for generating a surface acoustic wave(SAW). In this embodiment magneto-elastic energy conversion occursbetween a surface acoustic wave (SAW) in the piezoelectric layer and theexcitable layer of the oscillator is present, as shown in the device 300of FIG. 12. The piezoelectric layer 302 acts as the transport layer forthe surface acoustic wave 304, generated by at least one surfaceacoustic wave generating means 306. The latter may e.g. be aninterdigited electrode. The surface acoustic wave generating means 306is adjusted to generate in the transport layer a surface acoustic wave304 having a wavelength λ_(SAW) and having a frequency ν_(SAW). Thefrequency ν_(SAW) should be close to the peak frequency of thecurrent-induced oscillation (or an integer multiple thereof) such thatboth frequencies can lock to each other. Since the oscillator istypically of nano-scale dimensions, the wavelength of the SAW λ_(SAW)will be larger than the dimensions of the active part of the oscillator(e.g. the diameter of the nanohole). The interaction of the SAW 304 withthe ferromagnetic excitable layer 202 will therefore be quite uniformover the oscillator and entirely depend on the frequency, phasedifference and amplitude of the SAW 304 with respect to the peakfrequency of the oscillator. The transport layer comprises piezoelectricmaterial 302, and the ferromagnetic element may be in direct contactwith the transport layer or with the surface acoustic wave generatingmeans 306. The frequency of the surface acoustic waves may be chosen ina narrow frequency range around the ferromagnetic resonance frequency ormultiple thereof. The oscillation peak is preferably as narrow aspossible, as is represented by a high quality factor Q. The frequencyrange preferably used may then be the range with a width correspondingto a certain fraction of the width of the oscillation peak. Thisfraction may for example be 200%, 150%, 100%, 50%, 25%, 10%, 2% or 1%.

In a seventh embodiment, the invention relates to a device as describedin the previous embodiments, particularly embodiment 2, but allowing tocontrollably tune and reversibly alter the frequency of the generatedoscillation by indirect interaction with the excitable layer through themeans for generating the magnetic bias field. The means that is used totune the frequency may comprise a piezoelectric material or suspendedstructure that can apply a stress upon e.g. an anti-ferromagneticelement, e.g. layer, that is in contact with the excitable layer in acontrolled way. The piezo-electric layer can be in direct contact or notin direct contact with anti-ferromagnetic element, and when subject toan electrical field, the piezo-electric material will apply a stress onthe piezo-electric layer as well as on the anti-ferromagnetic element.The exchange bias field in a spin valve thus can be altered by theapplication of stress. Since the exchange bias field in the presentinvention is used to bias the ferromagnetic excitable layer 202, achange of the exchange bias field on application of stress will alsoalter the frequency of the oscillator. The means that is used to alterthe frequency may also change the magneto-static coupling between anadditional ferromagnetic element 352 and the ferromagnetic excitablelayer 202. The means that is used to change the magnetostatic couplingmay comprise a means 354, such as e.g. a piezoelectric material orsuspended structure, that can change the geometric distances between theadditional ferromagnetic element 352 and the ferromagnetic excitablelayer 202 and can therefore change the magnetostatic coupling. Aschematic illustration of this embodiment is shown in FIG. 13 a and FIG.13 b. Since the magnetostatic coupling provides the bias field, it setsthe frequency of the oscillation in the ferromagnetic excitable layer202, the frequency can be tuned by altering the magnetostatic coupling.

In a further embodiment of the present invention, the inventionfurthermore relates to a method for reading out a device generatingoscillations as described in the above described embodiments of thepresent invention. In the state-of-the-art, the magnetoresistance thatis present between the ferromagnetic excitable layer and theferromagnetic fixed layer and the variations of this magnetoresistancedue to the precessional motions of the excitable layer is responsiblefor the AC voltage at the output and thus is used as a reading out ofthe device. The present invention also relates to an alternative readout where the precessional motion that is excited by the spin polarisedcurrent changes another measurable parameter. The other measurableparameter may be for example a change of the resistance or voltage in alateral geometry. Possible physical effects at the origin of a lateralresistance change can be e.g. the anisotropic magnetoresistance of theferromagnetic excitable layer that is dependent on the direction of themagnetisation of the ferromagnetic excitable layer with respect to thedirection of the current through the layer. Another example of ameasurable parameter is the extraordinary Hall effect that is a measureof the magnetisation component orthogonal to the bias field and thecurrent sent through the excitable layer. The lateral resistance changecan be measured as an AC signal between at least two read out electrodes402, 404 that are in electrical contact with the ferromagnetic excitablelayer 202, as illustrated in FIG. 14 a The electrodes for providing thecurrent of spin polarised charge carriers are also shown in FIG. 14 a. Atop view is also provided for a system with two read out electrodes 402,404 in FIG. 14 b and for four read out electrodes 402, 404, 406, 408 inFIG. 14 c.

The alternative read out may further comprise an interlayer such as apiezoelectric material that converts the precessional motion of theexcitable layer into an electrical signal by using the magnetoelasticcoupling between the ferromagnetic excitable layer 202 and apiezoelectric layer 452. The latter is illustrated for device 450 inFIG. 15. It will be obvious to the person skilled in the art that otherreadout systems based on measurable parameters such as materialproperties, differing from the magnetisation, used to read-out thedevices as described above, are within the scope of the presentinvention.

Other arrangements for accomplishing the objectives of the device forgenerating a tunable oscillating signal embodying the invention will beobvious for those skilled in the art.

It is to be understood that although preferred embodiments, specificconstructions and configurations, as well as materials, have beendiscussed herein for devices according to the present invention, variouschanges or modifications in form and detail may be made withoutdeparting from the scope and spirit of this invention.

1. A device for generating an oscillating signal, the device comprising:means for providing a current of spin polarized charge carriers; amagnetic excitable layer adapted for receiving said current of spinpolarized charge carriers thus generating an oscillating signal with afrequency v_(osc); and an integrated means, different from said meansfor providing a current of spin polarized charge carriers, forinteracting with said magnetic excitable layer to thereby select saidoscillation frequency, wherein said interacting comprises performingmagnetic interactions comprising inducing mechanical stress in saidmagnetic excitable layer.
 2. A device according to claim 1, wherein saidintegrated means for interacting with said magnetic excitable layer is ameans for controllable tunable interacting with said magnetic excitablelayer such that a controllable tuning of said oscillation frequency isachieved.
 3. A device according to claim 1, wherein said magneticinteractions are interface interactions.
 4. A device according to claim1, wherein said interacting comprises performing any of magnetostaticinteractions and exchange bias interactions.
 5. A device according toclaim 1, wherein said magnetic excitable layer is a ferromagneticsemiconductor layer and said interacting comprises applying an electricfield over said ferromagnetic semiconductor layer.
 6. A device accordingto claim 1, comprising a means for generating a magnetic bias field tobias the magnetic excitable layer.
 7. A device according to claim 6,wherein said means for generating a magnetic bias field is anantiferromagnetic layer which is in at least partial magnetic contactwith said magnetic excitable layer.
 8. A device according to claim 7,comprising a means for generating stress upon said antiferromagneticlayer.
 9. A device according to claim 6, wherein said means forgenerating said magnetic bias field comprises an element offerromagnetic material that is magnetostatically coupled to saidmagnetic excitable layer.
 10. A device according to claim 9, furthercomprising a means for changing the geometric distances between saidmagnetic excitable layer and said ferromagnetic element.
 11. A deviceaccording to claim 10, wherein said means for changing the geometricdistances consists of one of a piezoelectric layer and a suspendedstructure.
 12. A device according to claim 1, wherein said integratedmeans for interacting with said magnetic excitable layer comprises aninteracting layer that is coupled via one of magneto-elastically,magneto-statically and exchange bias effect to said magnetic excitablelayer.
 13. A device according to claim 12, wherein said interactinglayer is a piezoelectric layer.
 14. A device according to claim 12,wherein said interacting layer is an antiferromagnetic layer.
 15. Adevice according to claim 12, further comprising a surface acoustic wavegenerating means that can generate a Surface Acoustic Wave in saidinteracting layer.
 16. A device according to claim 15, wherein saidinteracting layer is a structural part of the Surface Acoustic Wavegenerating means.
 17. A device according to claim 15, wherein saidsurface acoustic wave generating means generates a Surface Acoustic Wavein said interacting layer that has a frequency essentially equal to amagnetic resonance frequency of said excitable layer, or an integermultiple thereof.
 18. A device according to claim 12, wherein at leasttwo electrodes are provided on one of a surface and an inside of saidinteraction layer, which induces stress in said interaction layer byputting an electrical potential difference over them.
 19. A deviceaccording to claim 12, comprising a means for generating stress in saidinteraction layer by one of physical force and pressure build up.
 20. Adevice according to claim 1, wherein said means for providing a currentof spin polarized charge carriers is abutting on said magnetic excitablelayer and comprises an electrode, a spin polarization means and acurrent confinement structure.
 21. A device according to claim 20,wherein said means for providing a current of spin polarized chargecarriers comprises a fixed layer with a constant magnetic polarizationthrough which the current is passing, before entering into the excitablelayer.
 22. A device according to claim 21, wherein the fixed layer andexcitable layer are separated by an interlayer to magnetically separateboth layers.
 23. A device according to claim 1, further comprising areadout structure that measures excitation caused by the spin polarizedcurrent passing through said magnetic excitable layer.
 24. A deviceaccording to claim 1, further comprising a readout structure thatmeasures magneto-resistance generated by a combination of the fixedlayer and the magnetic excitable layer.
 25. A device according to claim1, further comprising a readout structure that comprises a piezoelectricmeasurement layer that converts precessional movement of the excitablelayer into an electrical signal.
 26. A device according to claim 1,further comprising a readout structure that measures resistance changeby measuring an AC signal between at least two electrodes in electricalcontact with said excitable layer.
 27. A device according to claim 1,further comprising a readout structure that measures change of one ofresistance and voltage in a lateral geometry.
 28. A method forgenerating oscillations, the method comprising; providing a current ofspin polarized charge carriers, thus generating an oscillating signalwith an oscillation frequency v_(osc) by interaction between saidcurrent of spin polarized charge carriers and a magnetic excitablelayer; and controllably tuning said oscillation frequency v_(osc) byinducing an interaction between an integrated means, different from saidmeans for providing a current of spin polarized charge carriers, andsaid magnetic excitable layer, wherein said interaction comprisesperforming magnetic interactions comprising inducing mechanical stressin said magnetic excitable layer.
 29. A method according to claim 28,wherein inducing an interaction between an integrated means and saidmagnetic excitable layer comprises any of inducing mechanical stress insaid magnetic excitable layer, inducing exchange bias interactions andinducing magneto static interactions.
 30. A method according to claim28, said magnetic excitable layer being a ferromagnetic semiconductorlayer, wherein inducing an interaction is performed by applying anelectric field over said ferromagnetic semiconductor layer.
 31. A methodfor reading out a magnetic element, the method comprising: providing acurrent of spin polarized charge carriers, thus generating anoscillating signal with an oscillation frequency v_(osc) by interactionbetween said current of spin polarized charge carriers and a magneticexcitable layer; controllable tuning said oscillation frequency v_(osc)by inducing an interaction between an integrated means, different fromsaid means for providing a current of spin polarized charge carriers,and said magnetic excitable layer, wherein said interaction comprisesperforming magnetic interactions comprising inducing mechanical stressin said magnetic excitable layer; and measuring an excitation caused bysaid spin polarized charge carriers.